Magnesium aluminate anion exchangers

ABSTRACT

Preparations and uses are shown for novel crystalline aluminates which conform generally to the empirical formula 
     
         MgA.sub.a.sup.V Z.sub.b.sup.v ·nAl(OH).sub.3 ·mH.sub.2 O 
    
     where 
     A and Z represent negative-valence ions or radicals selected from the group comprising hydroxyl, halide, inorganic acid, and organic acid, 
     n is a value of from about 1 to about 2, 
     v is a negative valence of 1, 2, or 3, 
     a and b each have values of from zero to 2, with (va)+(vb) equal to 2, and with m being a value of zero or more.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of our copending Ser. No. 71,920 filed Aug. 31, 1979, now U.S. Pat. No. 4,243,555, which is a divisional of Ser. No. 939,544 filed Sept. 5, 1978, now U.S. Pat. No. 4,183,900, which is, itself, a divisional of Ser. No. 812,542 filed July 5, 1977, now U.S. Pat. No. 4,116,857.

BACKGROUND OF THE INVENTION

In our U.S. Pat. Nos. 4,116,857 and 4,183,900 it is shown that microcrystalline MgX₂ ·2Al(OH)₃, where X is halide, prepared as a dispersion in situ within a particulate macroporous anion exchange resin is useful in selectively removing Mg⁺⁺ values from brines. It is understood, of course, that the crystals may contain waters of hydration. Additional embodiments, preparations, and uses of the crystalline MgX₂ ·2Al(OH)₃ are the principal subjects of this application.

Aside from the many naturally-occurring aluminates, there are synthesized metal aluminates such as found, e.g., in U.S. Pat. Nos. 2,395,931, 2,413,184, 3,300,577 and 3,567,472.

SUMMARY OF THE INVENTION

Crystalline MgX₂ ·2Al(OH)₃ is prepared by reacting MgX₂ (where X=halide, especially chloride) with Al(OH)₃ at elevated temperature. The so-formed crystalline MgX₂ ·2Al(OH)₃ is contacted with alkali metal hydroxide (esp. NaOH) to form novel crystalline Mg(OH)₂ ·nAl(OH)₃ ·mH₂ O. The Mg(OH)₂ ·nAl(OH)₃ is reacted with negative radicals or anions to form novel crystalline MgAZ·nAl(OH)₃ ·mH₂ O compounds, where A and Z represent negative radicals or anions selected from the group comprising of hydroxyl, halide, inorganic acids, and organic acid as disclosed in greater detail below.

DETAILED DESCRIPTION OF THE INVENTION

A carrier or medium is provided with alumina hydrate, Al(OH)₃, dispersed therein. The Al(OH)₃ is then reacted with brine containing MgX₂ at elevated temperature to form crystalline MgX₂ ·2Al(OH)₃, where X is a halide, preferably chlorine. When the microcrystalline MgX₂ ·2Al(OH)₃ is formed and dispersed in situ in a particulate, reticular anion exchange resin, the composite is particularly useful in selectively removing Mg⁺⁺ values from brines (as per U.S. Pat. Nos. 4,116,857 and 4,183,900).

The crystalline MgX₂ ·2Al(OH)₃ when prepared as a free precipitate, when supported on a substantially inert substrate, or when supported by (or dispersed in) a cation exchange resin, is useful as an exchanger of negative radicals or anions, such as hydroxyl ions, halide anions, inorganic acid anions, or organic acid anions, thereby forming novel compounds of the general formula MgAZ·nAl(OH)₃ ·mH₂ O where A and Z are monovalent anions of the group consisting of hydroxyl ions, halide ions, inorganic acid anions, and organic acid anions.

Throughout this disclosure X is employed to denote halide ions in the crystalline MgX₂ ·2Al(OH)₃ which serves as the precursor to the crystalline Mg(OH)₂ ·nAl(OH)₃. The symbols A and Z are employed to denote anions which then replace part or all of the hydroxyl groups attached to Mg in the crystalline Mg(OH)₂ ·nAl(OH)₃. Any of these products may contain waters of hydration.

The present invention provides new and useful crystalline magnesium aluminates conforming generally to the empirical formula

    MgA.sub.a.sup.v Z.sub.b.sup.v ·nAl(OH).sub.3 ·mH.sub.2 O

where

A and Z represent negative-valence ions or radicals selected from the group comprising hydroxyl, halide, inorganic acid, and organic acid,

n is a value of from about 1 to about 2,

v is a negative valence of 1, 2, or 3,

a and b are each values of from zero to 2, with (va)+(vb) equal to 2, and with m being a value of zero or more.

Throughout this disclosure, the waters of hydration are not always shown in the empirical formulas, but unless the compounds have been dehydrated, some waters of hydration are most likely present.

Of special interest is the reaction of the crystalline MgX₂ ·2Al(OH)₃, where X is halide (especially chlorine), with NaOH in sufficient quantity at elevated temperature to form novel crystalline Mg(OH)₂ ·nAl(OH)₃ ·mH₂ O which is found to undergo exchange with anions to form novel crystalline MgAZ·nAl(OH)₃ ·mH₂ O compounds.

The so-formed crystalline MgAZ·nAl(OH)₃ compounds are useful as sources of the radicals or anions (A and/or Z) by chemically or thermally releasing such anions in a desired media. Also, the novel Mg(OH)₂ ·nAl(OH)₃ and MgAZ·nAl(OH)₃ compounds are useful in forming MgAl₂ O₄ spinel structures at very high temperatures.

As disclosed in our U.S. Pat. Nos. 4,116,857 and 4,183,900, referred to supra and incorporated herein by reference, the crystalline MgX₂ ·2Al(OH)₃ (X=halide) may be prepared within the reticules of a particulate anion exchange resin, e.g., by the following steps:

1. combine the resin with aqueous AlCl₃ to obtain penetration of the AlCl₃ into the reticules of the resin;

2. convert the AlCl₃ to Al(OH)₃ by reaction with, e.g., NH₄ OH, washing out extraneous material; and

3. add MgX₂ (e.g. MgCl₂) and heat at elevated temperature for a time sufficient to form crystalline MgX₂ ·2Al(OH)₃.

In the context of this present disclosure, the crystalline MgX₂ ·2Al(OH)₃ may also be obtained by alkaline precipitation of AlCl₃ in an aqueous carrier or medium. The aqueous medium may also contain other liquid or dissolved materials which may be tolerated so long as they do not prevent the precipitation of the Al(OH)₃ or do not contaminate the Al(OH)₃ to such an extent that subsequent formation of reasonably pure MgX₂ ·2Al(OH)₃ is substantially deterred.

The aqueous media may also contain non-dissolved substances, such as ion exchange resins (cationic or anionic), inorganic particles, or organic polymers which may be reticular or substantially non-reticular, and which may be found to be beneficial so long as they are substantially inert with respect to the reactions involved in the formation of Al(OH)₃ or of the subsequent formation of crystalline MgX₂ ·2Al(OH)₃. These non-dissolved substances beneficially serve as substrates onto which (and into which) the MgX₂ ·2Al(OH)₃ crystals may form. These non-dissolved substances, which shall be referred to herein as "substrates", may be porous or non-porous ion exchange resins (anionic or cationic), may be organic (including polymers), or may be inorganic such as stable metal oxides. The amount of crystalline MgX₂ ·2Al(OH)₃ which may be deposited on a given weight of substrate is largely dependent on the surface area of the substrate, therefore it is generally best if the substrate is in finely divided form. It is evident, of course, that the substrate selected should be one which will withstand the heating in steps involved in the formation of the MgX₂ ·2Al(OH)₃ and withstand any subsequent desired reactions without interfering to any substantial extent with the desired reactions. The substrate, then, should be one which is substantially inert and substantially stable in the operation of the present invention. In those instances where the MgX₂ ·2Al(OH)₃ is converted to MgAZ·nAl(OH)₃ and/or Mg(OH)₂ ·nAl(OH)₃ which, in turn, is to be converted at high temperature to spinels, then selection of substrate, if any, requires consideration of the effect of high temperature on such substrate.

The following examples are to illustrate the practice of the present invention, but the invention is not limited to the particular embodiments illustrated.

EXAMPLE 1

An amorphous form of Al(OH)₃, 0.15 mol quantity, is precipitated by adding NH₄ OH to an aqueous solution of AlCl₃. To the so-formed Al(OH)₃ slurry is added 0.6 mole of MgCl₂. The mixture is heated to 70° C., adding NH₄ OH as needed to hold the pH up to 6.7 or more, then filtered and washed with NH₄ OH, then H₂ O. The so-formed MgCl₂ ·2Al(OH)₃ is redispersed with 5% aqueous solution of MgCl₂ (to assure more complete conversion of the Al(OH)₃) and heated at 95° C. overnight. Then filter, wash ppt. on filter with equimolar mixture of NH₄ Cl/NH₄ OH, then with NH₄ OH. Crystallinity of MgCl₂ ·2Al(OH)₃ is confirmed by x-ray diffraction pattern.

To the above product is added NaOH (1 N solution) to pH 11, and heated to 80° C. with stirring to disperse the solids. This is filtered, the solids combined with more NaOH (1 N solution) to pH 11, heated to 80° C., and refiltered. The NaOH reaction step is repeated 3 times as a precaution to assure substantially complete conversion to Mg(OH)₂ ·2Al(OH)₃. Then the filtered material is redispersed in H₂ O, heated, and filtered, then re-diluted with H₂ O and heated at 95° C. overnight to assure substantially complete crystallization. Upon passing the slurry through a mixed bed of Dowex 50-H⁺ and Dowex 1-OH⁻ ion exchange resins ("Dowex" is a Tradename of The Dow Chemical Company) on a coarse glass fritted filter, the slurry particles are found to be fine enough to pass through. The pH at this point is about 10.4. The slurry is evaporated to 400 ml. volume and by titration of 10 ml. with N/1 HCl is found to contain 1.22 mmol Al per 10 ml. of slurry.

Portions of the so-formed stable dispersion of crystalline Mg(OH)₂ ·2Al(OH)₂ (confirmed by x-ray diffraction pattern) are found to be neutralized by the following acid ions: Cl⁻, ClO₄ ⁻, H₂ PO₄ ⁻, HCrO₄ ⁻, SO₄ ⁻⁻, NO₃ ⁻, HPO₄ ⁻⁻, and the anion of 1-ascorbic acid, acetic acid, citric acid, and oxalic acid.

The above monovalent acid anions are represented by Z in the empirical formula Mg(OH)Z·nAl(OH)₃, the divalent anions are represented by Z in the empirical formula MgZ·nAl(OH)₃ and the trivalent anions are represented by Z in the empirical formula MgZ_(2/3) ·nAl(OH)₃. When all the OH ions are replaced by monovalent anions the empirical formula is MgZ₂ ·nAl(OH)₃. In MgAZ·nAl(OH)₃, A and Z may be the same monovalent ions or different monovalent ions.

EXAMPLE 2

AlCl₃ (0.28 moles) in 900 ml. H₂ O is precipitated as Al(OH)₃ by addition of NH₄ OH. MgCl₂ (1.0 mole) in water is added, with enough NaOH added to bring pH to 6.7 (neutral or barely basic). Heating the resulting ppt, at 70° C. causes crystallization. The MgCl₂ ·2Al(OH)₃ crystals are filtered out, washed with dil. NH₄ OH, then with H₂ O. Enough MgCl₂ (5% in H₂ O) is added to moisten the ppt. to give excess MgCl₂. Overnight heating at 95° C. and NaOH neutralization assures substantially complete crystallization (confirmed by x-ray) of Mg(OH)₂ ·2Al(OH)₃. Demineralize with Dowex-50 cation exchange resin to take out excess Cl⁻. At this point the fine particles of Mg(OH)₂ ·2Al(OH)₃ are easily dispersed in water to form a dispersion.

The Mg(OH)₂ ·2Al(OH)₃ is converted to Mg(OH)(HCO₃)·2Al(OH)₃ by reaction with CO₂. The x-ray pattern is essentially the same.

EXAMPLE 3

Mix 625 gms. of aqueous slurry containing 2.15 mol. amorphous Al(OH)₃ with 172 gms. of aqueous slurry containing 1.075 mol of Mg(OH)₂ (brucite) and add 74 mmol of NaOH (as 50% aqueous solution). Warming to 70° C. causes thickening; add 200 ml. of H₂ O to thin and heat to 95° C. in oven for 16 hours. Resulting thick paste is stirred into water. A sample of the slurry is filtered, washed and analyzed by x-ray and it is found that Mg(OH)₂ ·2Al(OH)₃ is the major crystalline component with a trace of bayerite (or gibbsite) and some amorphous Al(OH)₃ and Mg(OH)₂. The remaining slurry is put back into 95° C. for 24 hours and x-ray shows more crystalline Mg(OH)₂ ·2Al(OH)₃ with a small amount of boehmite.

A small disc (about 22 mm diameter and 7.5 mm thick) when pressed and fired at high temperature (>1200° C.) is found to have formed spinel, MgAl₂ O₄, and to have a density of about 94% of theoretical.

There are, of course, many organic acids and inorganic acids which contain anions which are operable in the present invention. The larger the anion, the greater is the possibility of expanding the crystal lattice structure to the limit, while still maintaining the crystallinity. For example, with 1-ascorbic acid (vitamin C) which is a 6-carbon molecule, the Mg(OH)Z·2Al(OH)₃ crystal is considerably expanded by the large anion (Z) which has replaced (exchanged with) one of the OH⁻ ions. Other organic acids having from 1 to about 8 carbon atoms are within the purview of this invention, including monobasic, dibasic, and tribasic carboxylic acids.

The various novel compounds of the present invention may be used in forming spinels, MgAl₂ O₄, at high temperature and are also useful in exchanging with other anions in various solutions. For instance, ascorbic acid groups may be released from the present crystalline compounds into solutions which contain other anions, such as Cl⁻, which can replace the ascorbic acid in the crystal. 

We claim:
 1. A method for removing negative-valent ions or radicals from aqueous medium, said method comprising contacting said aqueous medium with crystalline Mg(OH)₂ ·nAl(OH)₃ ·mH₂ O, where n is a value of from about 1 to about 2 and m is a value of zero or more,thereby exchanging OH ions in the said crystalline material with the said negative-valent ions or radicals. 